The present invention is a Dynamic Laser Speckle Profilometer (DLSP) apparatus and a dynamic laser speckle profilometer method, to preform nondestructive analysis of materials, components, and assemblies by creating an optoelectronic phase map leading to the deformation and resonance mode mapping of an object under test.
This invention relates to interferometers of the kind comprising an optical imaging system disposed optically between an object location and an image location. There is a means for producing a beam of coherent light with separate object and reference beams respectively directed to the object and image locations. In this manner, when a light-scattering surface is disposed at the object location so as to be illuminated by the object beam and a screen is disposed at the image location so as to be illuminated by the reference beam, light from the object beam scattered by said surface will be imaged on the screen. The imaging system will cause interference at the screen with the light of the reference beam and the scattered object beam.
Interferometers of the kind specified are particularly adapted for use in inspection systems employing the techniques of electronic speckle pattern interferometry.
Using an interferometer of the kind specified, a surface to be inspected is disposed at the object location, and disposed at the image location is a photo-sensitive screen of a television camera device, such as a vidicon tube. There is a means to derive a video signal representing the point-by-point variations of intensity in the resultant pattern of illumination formed on the screen. Because it is partly formed by the imaging of scattered coherent light, this pattern of illumination exhibits the phenomenon known as "speckle effect". By virtue of the form of the reference beam, the range of spatial frequencies in the resultant pattern of illumination will not extend materially beyond the range of spatial frequencies in the "speckle pattern". The aperture should be made sufficiently small to ensure that the spatial frequencies in the pattern of illumination formed on the screen lie wholly or mainly within the range which can be resolved by the television camera device. As a typical example, an aperture of f/16 may suitably be used when the television camera device is a standard 2.5 cm vidicon tube capable of resolving 600 picture lines.
In prior British Patent Specification no. 1,392,448, of the National Research Development Corporation an optical inspection system is provided which uses speckle pattern for optical inspection by correlating two similar video signals representing point by point variations of intensity in two patterns of illumination from the irradiation of a photosensitive screen with light from first and second interfering beams, one of which is a pattern of illumination from light scattered from the surface of an object being inspected. A related British Patent Specification No. 1,460,861 by the same company uses the same technique but light imaged by the imaging system is arranged to be reflected to the image location by a planar mirror which also serves as a spatial filter for the second beam. In a third related British specification no. 1,593,284 an Optical Inspection system using the same speckle interferometer technique and deriving from a video screen a video signal representing the spatial variations in the sum of the intensities in two patterns of illumination independently on the screen.
Aside from the basic laws of physics for optics which dictate the criteria for the "speckle effect", the present invention goes beyond the invention taught in these British Patents by creating a new low noise optical design with the capabilities of making continuous or pulsed measurements, modulating the phase of the reference beam, phase tracking the relative phase shift caused by the motion of the object under test with respect to the DLSP by incorporating a laser range finder. This laser range finder is coupled to a phase tracker in the reference beam. The entire system is controlled through a central CPU to produce optoelectronic phase maps of the object under test which can then be used by the system to produce deformation maps, resonance maps, displacement maps, stress and strain maps, bending moment maps. All of which, can further be used by the system for finite elemental analysis, mode analysis, deformation profilometry, and resonance amplitude analysis.